In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must assess the structural integrity of composite materials. This assessment detects inclusions, delaminations and porosities. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One solution uses an ultrasonic source to generate ultrasonic surface displacements in a work piece which are then measured and analyzed. Often, the external source of ultrasound is a pulsed generation laser beam directed at the target. Laser light from a separate detection laser is scattered by ultrasonic surface displacements at the work piece. Collection optics then collect the scattered laser energy. The collection optics are coupled to an interferometer or other device, and data about the structural integrity of the composite structure can be obtained through analysis of the scattered laser energy. Laser ultrasound has been shown to be very effective for the inspection of parts during the manufacturing process. However, the equipment used for laser ultrasound must be precisely calibrated to obtain accurate measurements. Calibration of optical components requires placing calibration targets at specific locations in the inspection field, aiming and focusing the optical systems at the target, and recording optical system data such as lens position, azimuth and elevation. The target is then moved to other locations in the inspection field, and the process is repeated. After repeating the process at multiple locations within the inspection field, algorithms can correlate the camera position in the inspection field.
Additional optics and sensors, such visual cameras, depth cameras, and other like sensors can establish the position and orientation of the work piece. Highly accurate measurements require calibration of the multiple sensors and the ability to correlate measurements to the work piece or final world framework.
Single-mode targets, when used to calibrate equipment, require a different target to calibrate each optical component. This is because each optical component may operate at different wavelengths. A single-mode target, designed to work for one optical sensor, will not function properly as a target for a different optical system operating at a different wavelength. For example, a flat target having a contrasting pattern can serve as a suitable target for an optical camera, but will not serve as a suitable target for a depth camera. The flat target provides no depth contrast. Calibrating each optical sensor independently with a single-mode target is labor intensive, time consuming and can introduce intersection errors in the calibration process. Errors introduced in the calibration process will result in imprecise measurements when the laser ultrasound system is operating in the final world framework.